1. Field of the Invention
The invention relates to an electric power steering apparatus.
2. Description of Related Art
Some conventional vehicle electric power steering apparatuses apply an assist force at the time of a steering operation by reducing the rotation speed of an electric motor by use of a speed reduction mechanism and then transmitting the resultant rotation to a steering system, for example, as disclosed in Japanese Patent No. 4491716 (JP 4491716).
The speed reduction mechanism includes a worm, a worm wheel, and a driving shaft to which the worm is attached. The driving shaft is rotatably supported by a housing via a pair of rolling bearings provided at respective opposite ends of the driving shaft. A shaft coupling is disposed between an output shaft of the electric motor and the driving shaft to transmit rotation.
The shaft coupling includes a first yoke attached to the output shaft, a second yoke attached to the driving shaft, and an intermediate member disposed between the first yoke and the second yoke. The first yoke has a cylindrical first boss portion, and a plurality of pawl portions protruding toward the intermediate member with respect to the first boss portion and having a trapezoidal shape. The second yoke has a cylindrical second boss portion, and a plurality of pawl portions protruding toward the intermediate member with respect to the second boss portion and having a trapezoidal shape. The intermediate member has a cylindrical intermediate boss portion, a plurality of arm portions extending radially from the intermediate boss portion, and trapezoidal engagement spaces each defined by a pair of the adjacent arm portions.
The intermediate member engages with the first yoke such that the first boss portions are placed in every other engagement space among the engagement spaces provided in a circumferential direction, and the intermediate member engages with the second yoke such that the second boss portions are placed in the remaining engagement spaces. Thus, the rotation of the output shaft is transmitted to the driving shaft via the first yoke, the intermediate member, and the second yoke.
In the shaft coupling configured as described above, a stick-slip sound may be generated in the shaft coupling portion while the output shaft, the shaft coupling, and the driving shaft are in rotation.